Voltage drop is normal when you remove the grid charger and let pack sit, you won't discover anything about the condition doing that. You should get the IMA trouble codes/blink codes, here's a post on how to get the blink codes: Misc. RESETS...The IMA light is on and after letting it grid charge for a little while and turn off the grid charger I can watch the volts drop on the IMA.
Can't think of any IMA codes that disable 12V charging, off the top of my head, but with a really bad or discharged pack 12V charging will disable (but 12V charging should work if you bypass the pack entirely, by turning the pack switch off and disconnecting connectors on the battery computer). If you know for a fact you have a bad 12V battery, probably best to get the new one in, check the 3 grounds while you're there, make sure they're solid, then get the IMA pack sorted.